Minimal XML representation of Penpot shapes

[opcode81]

From the Penpot SVG export, determine the minimal XML representation, removing the actual SVG content which is not used by Penpot when importing the format.
This representation could serve as the basis for variation generation (#68).

[opcode81]

We definitely need to retain

• all elements in the penpot namespace
• <g> tags that are immediate parents of <penpot:shape> elements

[opcode81]

There are penpot shapes for which the SVG representation must be retained:

• A penpot shape with type="path" requires the shape's <g> sibling and the contained path element for the import to work.
• Similarly, a penpot shape with type="circle" require's the shape's <g> sibling and the contained ellipse element.

This gives rise to a simplified conversion rule:
It appears that a penpot shape's <g> sibling must be retained unless the subsequent sibling is a penpot element.

Examples:

<g>
   <penpot:shape type="bool">...</penpot:shape>
   <defs/>
   <g>...</g>   <!-- can be removed -->
   <penpot:bool>...</penpot:bool>
</g>

<g>
   <penpot:shape type="circle">...</penpot:shape>
   <defs/>
   <g>...</g>   <!-- must be retained -->
</g>

Problem: redundant representation

While the conversion rule is simple enough, it's highly unfortunate from a representational point of view, because it makes the representation quite redundant.
Example with a circle:

<g id="shape-459cc494-9782-8033-8004-753e7ef8b63c">
	<penpot:shape penpot:name="Circle-6" penpot:type="circle" penpot:transform="matrix(1.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000)" penpot:transform-inverse="matrix(1.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000)" penpot:proportion="1" penpot:proportion-lock="false" penpot:rotation="0" penpot:center-x="1172" penpot:center-y="510" penpot:constraints-h="scale" penpot:constraints-v="scale" viewBox="0.0 0.0 0.0 0.0">
		<penpot:svg-import/>
		<penpot:fills>
			<penpot:fill penpot:fill-color="#b1b2b5" penpot:fill-opacity="1"/>
		</penpot:fills>
	</penpot:shape>
	<defs/>
	<g class="fills" id="fills-459cc494-9782-8033-8004-753e7ef8b63c">
		<ellipse cx="1172" cy="510" rx="36" ry="36" transform="matrix(1.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000)" style="fill:#b1b2b5;fill-opacity:1"/>
	</g>
</g>

In this example, the penpot:shape element of type circle already contains all the information in the ellipse tag, except, oddly enough, the radius. Yet the Penpot import fails if the <g> and containing <ellipse> are removed.